How do I batch import one format, then export to another?
Here is an outline:


Make sure 3DS importer and Three.js exporter are installed and enabled

# test
hasattr(bpy.types, "EXPORT_OT_threejs")

# enable
try:
    bpy.ops.wm.addon_enable(module="threejs")
except ImportError:
    print("Threejs importer addon not available")

Loop over all the files and perform below actions for every single one,
you may use glob.glob() or os.walk() unless you want to select the files via UI
(see the Operator Import tempate, you will need to provide
files = bpy.props.CollectionProperty(type=bpy.types.PropertyGroup), docs)
Clear scene if desired, e.g.

 bpy.ops.object.mode_set(mode='OBJECT')
 bpy.ops.object.select_all(action='SELECT')
 bpy.ops.object.delete()

Import 3DS, basically call the operator bpy.ops.import_scene.autodesk_3ds(filepath="the_path_to_file.3ds")
The importer should select the imported files, but there will usually not be an active object. To run operations on the imported data or for some exporters you might need to set one object active (or process each individually and make one active after another).
Export to JS, basically call the operator bpy.ops.export.threejs(filepath="target_file.js", **kwargs),
kwargs stands for all additional arguments you may wanna give to the export operator.